Certain fibers are very difficult to cut. For example, Kevlar 49 (DuPont's trademark for one of its aramid fibers), is so hard (low modulus of elasticity) that, pressed against tool steel, the fiber embosses the steel. In addition, distribution of fibers along a blade (across the width of a moving belt), as occurs in cutting randomly oriented strands of waste material, will, due to the fiber hardness, subject those portions of the blade supported by a greater number of fibers (supported in turn by an anvil under the belt) to stresses causing harm to the blade edge. When making shear cuts using a blade and bed knife, the random strand orientation already mentioned means that some strands lie more nearly normal to the shear plane, while others lie more nearly parallel to it, again causing large localized stresses, harmful here to both blade and bed knife. High interfiber friction inhibits transverse (in the direction of blade thickness) sliding responsive to the horizontal component of the force imposed by the V-edged blades, which aggravates the cutting problem by increasing the peak cutting forces needed and heating of the fibers.
Prior efforts to cut such materials, using hardened steel or carbide blades and bed knives, and conventional anvils, have failed. The anvils have become embossed by the fibers, and the blade and bed knife edges have been broken down. And with thermoplastic fibers, the heat created by interfiber friction has fused fibers.